Understanding the psychological factors that influence investment decisions is crucial for predicting stock investment retention. This study investigates the mediating role of the Big Five personality traits in stock investment retention, utilizing a modified version of the theory of planned behavior. By examining the influence of investors’ perceived risk and attitudes toward stock investment, data collected via an online survey with The Association of Thai Securities Companies (ASCO) were analyzed using Structural Equation Modeling (SEM). The findings reveal that extraversion, openness, and conscientiousness significantly impact attitudes toward stock investing, which in turn affects investment retention. However, personality traits do not directly influence risk perception. This research provides unique empirical evidence of the independence between the Big Five personality traits and risk perception among Thai stock investors, underscoring the importance of personality in shaping investment behavior through its effect on attitudes.